1. Technical Field of the Invention
The present invention relates generally to wireless communication systems and, more particularly, to low noise amplifiers in such wireless communication systems.
2. Description of Related Art
Mobile communication has changed the way people communicate and mobile phones have been transformed from a luxury item to an essential part of every day life. The use of mobile phones today is generally dictated by social situations, rather than being hampered by location or technology. While voice connections fulfill the basic need to communicate, and mobile voice connections continue to filter even further into the fabric of every day life, the mobile Internet and moving video, including broadcast video, are the next steps in the mobile communication revolution. The mobile Internet is poised to become a common source of everyday information, and easy, versatile mobile access to this data will be taken for granted. Similarly, video transmissions to handheld user equipment will allow movies and television programs to be viewed on the go.
Third generation (3G) cellular networks have been specifically designed to fulfill many, if not all, of these future demands. As these services grow in popularity and usage, factors such as cost efficient optimization of network capacity and quality of service (QoS) will become even more essential to cellular operators than it is today. These factors may be achieved with careful network planning and operation, improvements in transmission methods, and advances in receiver techniques. To this end, carriers want technologies that will allow them to increase downlink throughput and, in turn, offer advanced QoS capabilities and speeds that rival those delivered by cable modem and/or DSL service providers. In this regard, networks based on Wideband Code Division Multiple Access (WCDMA) technology may make the delivery of data to end users a more feasible option for today's wireless carriers.
The General Packet Radio Service (GPRS) and Enhanced Data rates for GSM (EDGE) technologies may be utilized for enhancing the data throughput of present second generation (2G) systems such as GSM. The Global System for Mobile telecommunications (GSM) technology may support data rates of up to 14.4 kilobits per second (Kbps), while the GPRS technology may support data rates of up to 115 Kbps by allowing up to 8 data time slots per time division multiple access (TDMA) frame. The GSM technology, by contrast, may allow one data time slot per TDMA frame. The EDGE technology may support data rates of up to 384 Kbps. The EDGE technology may utilizes 8 phase shift keying (8-PSK) modulation for providing higher data rates than those that may be achieved by GPRS technology. The GPRS and EDGE technologies may be referred to as “2.5G” technologies.
The Universal Mobile Telecommunications System (UMTS) technology with theoretical data rates as high as 2 Mbps, is an adaptation of the WCDMA 3G system by GSM. One reason for the high data rates that may be achieved by UMTS technology stems from the 5 MHz WCDMA channel bandwidths versus the 200 KHz GSM channel bandwidths. The High Speed Downlink Packet Access (HSDPA) technology is an Internet protocol (IP) based service, oriented for data communications, which adapts WCDMA to support data transfer rates on the order of 10 megabits per second (Mbits/s). Developed by the 3G Partnership Project (3GPP) group, the HSDPA technology achieves higher data rates through a plurality of methods.
Where HSDPA is a downlink protocol, High Speed Uplink Packet Access (HSUPA) technology addresses the uplink communication. HSUPA is also specified by the 3GPP group to provide a complement data link to HSDPA. HSUPA also offers broadband IP and is based on software. HSUPA also extends the WCDMA bit rates, but the uplink rates may be less than the downlink rates of HSDPA. Where prior protocols severely limited the uplink connections, HSUPA allows for much higher uplink rates.
Likewise, standards for Digital Terrestrial Television Broadcasting (DTTB) provide for transmission of broadcast video. Three leading DTTB systems are the Advanced Television Systems Committee (ATSC) system, the Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) system, and the Digital Video Broadcasting (DVB) system, which includes terrestrial transmission under Digital Video Broadcasting-Terrestrial (DVB-T) specifications and transmissions to handheld devices under Digital Video Broadcasting-Handheld (DVB-H) specifications. DVB-H is an adaptation of DVB-T to handheld units, in which additional features are implemented to meet specific requirements of handheld units. DVB-H allows downlink channels with high data rates and may be made as enhancements to current mobile wireless networks. DVB-H may use time slicing technology to reduce power consumption of handheld devices.
A number of constraints are encountered with mobile units, so that systems (such as DVB-H) that communicate with mobile units typically need to address these constraints. Mobile users with small handheld units are difficult to target and the environment is constantly changing as the user's physical location changes. For example, a user may move from one cell sector to another. For the system, the number of users in a given broadcast area, such as a cell sector, may change considerably. Since most handheld devices are battery powered, power consumption in the handheld unit is a significant concern. Accordingly, in the design of handheld units, it would be advantageous to consider some of these constraints for receiving transmitted signals, such as DVB-H signals.
For a wireless communication device to participate in wireless communications, it typically includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). The transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with a local oscillator signal to produce radio frequency (RF) signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
In some instances where two-way communication is not necessary, the wireless communication device may only contain a receiver to receive signals. A handheld device, such as that used for reception of broadcast radio (AM/FM) or television, may not need the two-way communication. Hence, for reception only, the wireless device need only have a receiver to receive the broadcast signal. Accordingly, a DVB-H handheld device, may only have a receiver in the device.
The receiver is coupled to an antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies them. The one or more intermediate frequency stages mix the amplified RF signals with a local oscillator signal to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
One of the common components in a receiver RF front end is the low noise amplifier (LNA). Because the received RF signal is weak, LNA is used to boost (amplify) the signal without introducing appreciable additional noise. General communications receiver technology involves the use of a LNA for a particular RF front end. However, as integration increases to place more circuitry onto a single integrated circuit chip (system-on-chip or SoC) and such SoC components are placed into small handheld devices, a more robust LNA designs may be needed. For example, increased integration of RF along with digital processor circuitry in SoCs, increases noise and interference levels in the RF systems. Therefore, increased levels of noise immunity may be required from the RF part of the SoC to provide the required performance.
To increase noise immunity, typical integrated LNAs (and for that matter all analog and RF circuits in an integrated receiver) are differential, i.e. the input is applied as a differential signal and all inputs and outputs are also differential signals. Since typical antennas in communication systems are single-ended, either internal or external single-ended to differential conversion subcircuits are used, which can be either passive or active circuits. External passive single-ended to differential transformers have the advantage of better performance but add cost and size to the receiver implementation. Topologies where the LNA has a single-ended input and differential output avoid using external components, but depending on the environment, they could be prone to performance degradation due to noise (for example, operation in a device attached to a PC). Noise from neighboring devices at the input of a differential LNA is usually applied at both the P and N pins of the differential input, thus being cancelled by the common mode rejection of the differential amplifier. In single-ended input LNAs this mechanism does not exist, thus noise at the input can be detrimental.
Accordingly, there exists the need for a re-configurable LNA which, depending on the environment, can operate either as single-ended or differential. The selection between the two modes may be done in software and possibly using different external matching components. In addition, there are situations where the desired input signal power is high enough and the performance of a differential LNA is not necessarily needed. In that case a single-ended LNA may have adequate performance, as well as the advantage of lower power consumption. It would be beneficial, especially from the power consumption point of view, to have a receiver that can dynamically use either a single-ended or a differential LNA, which may be made depend on the overall receiver performance.